Follow-up the PR #4278 (Fixups for PID_ADD_EXTRUSION_RATE and HOTENDS==1)

Marlin/configuration_store.cpp

   #endif
 
   #if ENABLED(PIDTEMP)
-    #if ENABLED(PID_PARAMS_PER_HOTEND)
+    #if ENABLED(PID_PARAMS_PER_HOTEND) && HOTENDS > 1
       HOTEND_LOOP()
     #else
       int e = 0; UNUSED(e); // only need to write once

Marlin/temperature.cpp

 #endif
 
 #if ENABLED(PIDTEMP)
-  #if ENABLED(PID_PARAMS_PER_HOTEND)
+  #if ENABLED(PID_PARAMS_PER_HOTEND) && HOTENDS > 1
     float Temperature::Kp[HOTENDS] = ARRAY_BY_HOTENDS1(DEFAULT_Kp),
           Temperature::Ki[HOTENDS] = ARRAY_BY_HOTENDS1((DEFAULT_Ki) * (PID_dT)),
           Temperature::Kd[HOTENDS] = ARRAY_BY_HOTENDS1((DEFAULT_Kd) / (PID_dT));